The ratio of asymmetric to symmetric fission in fission of 239Pu by neutrons of energies from 30 keV to 14.7 MeV

Abstract

Radiochemical measurements have been made of the peak-to-valley ratio of the mass yield curve for neutron induced fission of 239Pu at neutron bombarding energies ranging from 30 keV to 14.7 MeV, and the results compared with similar measurements on 235U. For both target nucleides, the ratios all show a similar non-linear dependence on nuclear temperature at excitation energies below that at which intrinsic nucleonic levels become available. Once such levels are available, a more or less linear dependence on nuclear temperature is seen. This behaviour suggests that the mass asymmetry is governed primarily by the transition state spectrum, at least at low energies, and calculations have been made to check one of the experimentally measured ratios in 239Pu fission. The calculations are based on the α 3 symmetries of the levels of the transition state spectrum up to 300 keV, beyond which present knowledge of the spectrum is too sketchy to allow further calculation. This limited calculation agrees moderately well with the trend in the experimental results and so, as far as it goes, supports the applicability of the channel theory of fission to the mass division.